1. Field of the Invention
The present invention relates to a semiconductor laser used as a light source in an optical disc device and to a manufacturing method for such semiconductor laser.
2. Description of the Prior Art
Optical disc drives for digital video discs (DVDs) and other such media have been developed in recent years. Of the semiconductor lasers currently available, such devices mainly use AlGaInP-type semiconductor lasers that emit laser light of a short wavelength as their light sources.
FIG. 7 shows a cross-section of a real index guided-type laser. The expressions xe2x80x9cabovexe2x80x9d and xe2x80x9cbelowxe2x80x9d in the following explanation refer to the structure when FIG. 7 is in an upright position. The illustrated real index guided-type laser has an n-type GaAs substrate 1, on which an n-type GaAs buffer layer 2, an n-type cladding layer 3 made of Al0.35Ga0.15In0.5P, an active layer 4, a first p-type cladding layer 5 made of Al0.35Ga0.15In0.5P, and an etch-stop layer 6 made of (AlxGa1xe2x88x92x)0.5In0.5P (where 0xe2x89xa6xxe2x89xa60.1), AlzGa1xe2x88x92zAs (where 0.4xe2x89xa6zxe2x89xa61) or the like are successively formed in the stated order. A second p-type cladding layer 7 is then formed from Al0.35Ga0.15In0.5P as a ridge in the center of the upper surface of the etch-stop layer 6. An ohmic contact layer 8 made of p-type Ga0.5In0.5P is then formed on top of this second p-type cladding layer 7. A current-blocking layer 9 made of n-type Al0.35Ga0.15In0.5P is formed on both sides of the second p-type cladding layer 7 and the ohmic contact layer 8, and a cap layer 10 made of p-type GaAs is then formed on top of the ohmic contact layer 8 and the current-blocking layer 9. A p-type electrode 11 is formed on the cap layer 10, and an n-type electrode 12 is formed on the back of the n-type GaAs substrate 1. The second p-type cladding layer 7 and the current-blocking layer 9 form a light-confining construction, with light being confined within this and the n-type cladding layer 3. Note that the materials cited here are mere examples, so that other combinations of materials may be used.
Each layer in the AlGaInP-type semiconductor laser described above is successively formed using metalorganic vapor phase epitaxy (MOVPE). The light-confining construction is formed as follows. A material layer used to produce the second p-type cladding layer 7 is first provided on top of the etch-stop layer 6, an etching mask is applied to the part of the material layer that corresponds to the second p-type cladding layer 7, and an etching solution including sulfuric acid is applied to the unmasked parts of the material layer. This results in the unmasked parts being etched as far as the etch-stop layer 6, leaving a ridge of the material layer that forms the second p-type cladding layer 7. The etching mask is then removed, and the current-blocking layer 9 made of Al0.35Ga0.15In0.5P is formed through crystal growth using MOVPE. Before this current-blocking layer 9 is formed, however, impurities (which are mainly composed of the etching solution that remains after the etching process) need to be removed from the surface of the multilayer structure formed of the n-type GaAs substrate 1 to the ohmic contact layer 8. These impurities are removed by a thermal cleaning process where the multilayer structure is heated to a high temperature (generally 700xc2x0 C. or higher) that is near the crystal growth temperature of the current-blocking layer 9 that is formed next. To prevent phosphorous from being vaporized from the surface of the ohmic contact layer 8, a gas such as phosphine (PH3) is supplied during the heating.
An AlGaInP semiconductor laser used in an optical disc device needs to have improved laser characteristics in keeping with the optical disc device, which is to say, to oscillate in a unified lateral mode and to have a low threshold current. As a result, the thickness and shape of the first p-type cladding layer 5 and second p-type cladding layer 7 need to be appropriately determined, while the crystallization of the current-blocking layer 9 needs to be improved.
It is a first object of the present invention to provide a semiconductor laser that has improved laser characteristics, such as a lower threshold current.
It is a second object of the present invention to provide a manufacturing method for a semiconductor laser that has improved laser characteristics, such as a lower threshold current.
In order to achieve the stated objects, the inventors first examined the following aspects of conventional semiconductor lasers.
Firstly, the inventors studied why irregularities (bumps and concaves) appear in the surface of the etch-stop layer. When the etch-stop layer is formed of (AlxGa1xe2x88x92x)0.5In0.5P (where 0xe2x89xa6xxe2x89xa60.1), AlzGa1xe2x88x92xAs (where 0.4xe2x89xa6zxe2x89xa61) or the like, crystal growth has to be performed at a temperature of 700xc2x0 C. or higher to obtain a second p-type cladding layer and a current blocking layer with a high degree of crystallization. Thermal cleaning also has to be performed at a temperature of 700xc2x0 C. or higher. Due to these high temperatures, the (AlxGa1xe2x88x92x)0.5In0.5P (where 0xe2x89xa6xxe2x89xa60.1) or AlzGa1xe2x88x92zAs (where 0.4xe2x89xa6zxe2x89xa61) sublimates at the surface of the etch-stop layer. In order words, the heating performed when the second p-type cladding layer is formed using MOVPE causes sublimation which results in irregularities being formed in the surface of this layer. The heating performed when the current blocking layer is formed using MOVPE and the heating performed as part of the thermal cleaning also cause sublimation in the surface of the etch-stop layer, which also causes irregularities. The part of the structure forming the current blocking layer is subjected to high temperature at least twice during the manufacturing process, so that the irregularities in its surface are more prominent than those in the surface of the second p-type cladding layer. As a result, the second p-type cladding layer and the current blocking layer cannot be formed with a high degree of crystallization, which makes it impossible to produce a semiconductor laser with the desired characteristics. Through experimentation, the inventors found that the sizes of the irregularities in the surface of the etch-stop layer depend on the proportion of aluminum to gallium in the (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material forming the etch-stop layer.
A second phenomenon is the formation of a metamorphosed layer on the surface of the etch-stop layer. When the etch-stop layer is formed of (AlxGa1xe2x88x92x)0.5In0.5P (where 0xe2x89xa6xxe2x89xa60.1) or AlzGa1xe2x88x92zAs (where 0.4xe2x89xa6zxe2x89xa61) and thermal cleaning is performed in the presence of a gas such as phosphine (PH3) at a temperature of 700xc2x0 C. or higher, the (AlxGa1xe2x88x92x)0.5In0.5P (where 0xe2x89xa6xxe2x89xa60.1) or AlzGal1xe2x88x92zAs (where 0.4 less than z less than 1) surface of the etch-stop layer reacts with the phosphine or other gas, forming a metamorphosed layer. Also, crystal growth has to be performed at a temperature of 700xc2x0 C. or higher to obtain a second p-type cladding layer and a current blocking layer with a high degree of crystallization. When such high temperature is used, however, the surface of the etch-stop layer will absorb more of the impurities such as silicon that remain inside the reactor (while the reaction takes place after first evacuating the reactor, it is practically impossible to remove all such impurities) than are absorbed when a lower temperature is used. These absorbed impurities are one cause in the formation of a region (metamorphosed layer) of crystal defects. The part of the structure forming the current blocking layer is subjected to high temperatures at least twice, during which the part comes into contact with the impurities and phosphine gas, making the formation of a metamorphosed layer more evident for the surface of the etch-stop layer than for the surface of the second p-type cladding layer. In other words, when the second p-type cladding layer and current blocking layer are formed on top of an etch stop layer that has a metamorphosed layer formed on its surface, a high degree of crystallization cannot be achieved and a semiconductor laser with the desired characteristics cannot be produced. From experimentation, the inventors found that the extent to which a metamorphosed layer is formed on the surface of the etch-stop layer depends on the proportion of aluminum to gallium in the (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material forming the etch-stop layer.
The stated first object of the present invention can be achieved by a semiconductor laser, including: a first cladding layer; an active layer that is formed on top of the first cladding layer; a second cladding layer that is formed on top of the active layer and has a different type of conductivity to the first cladding layer; an etch-stop layer that is formed on top of the second cladding layer and has a same type of conductivity as the second cladding layer; and a light-confining construction that is formed on top of the etch-stop layer by an etching process, wherein the etch-stop layer has a surface part that contacts the light-confining construction, the surface part being composed of an (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor, where 0.2xe2x89xa6x less than 0.7 and 0 less than yxe2x89xa61.
In a semiconductor laser of this construction, the etch-stop layer has a surface part formed of an (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor (where 0.2xe2x89xa6x less than 0.7 and 0 less than yxe2x89xa61) that suffers from relatively little sublimation and is relatively reactive with various gases. As a result, sublimation of the surface of the etch-stop layer is suppressed when the layer is exposed to high temperatures as part of the thermal cleaning process and the process forming the layers composing the light-confining structure adjacent to the etch stop layer. The surface of the layer also does not react with the surrounding gas.atmosphere during these processes. For these two reasons, an extremely flat etch stop layer can be achieved, with no metamorphosed layer being formed on its surface. This makes it possible to form the layers composing the light-confining structure on top of the etch stop layer with no decrease in the degree of crystallization, and to produce a semiconductor laser with superior laser characteristics (threshold current, slope efficiency).
Here, the light-confining construction may include: a third cladding layer that is formed as a ridge on a specified region of a surface of the etch-stop layer and has a same type of conductivity as the etch-stop layer; and a current-blocking layer that has a different type of conductivity to the third cladding layer and is formed on both sides of the third cladding layer on regions of the surface of the etch-stop layer aside from the specified region.
Here, the etch-stop layer may have a band gap that is narrower than a band gap of the active layer.
In addition to the effects described above, light that is emitted by the active layer will be absorbed by the etch stop layer, so that it becomes unnecessary to provide a separate layer for absorbing this light. As a result, a self-excited oscillating laser (a pulse laser) can be achieved with a smaller overall structure.
The second object of the present invention can be achieved by a semiconductor laser manufacturing method, including: a first process forming (i) a first cladding layer, (ii) an active layer on top of the first cladding layer, and (iii) a second cladding layer on top of the active layer, the second cladding layer having a different type of conductivity to the first cladding layer; a second process for forming an etch-stop layer on top of the second cladding layer, the etch-stop layer having a same type of conductivity as the second cladding layer; a third process for forming a ridge-shaped third cladding layer on a specified region of a surface of the etch-stop layer using liquid-phase etching, the third cladding layer having a same type of conductivity as the etch-stop layer; a fourth process for performing thermal cleaning in a specified gas atmosphere after the third process has finished; a fifth process for forming a current-blocking layer on both sides of the third cladding layer on regions of the surface of the etch-stop layer aside from the specified region, the current-blocking layer having a different type of conductivity to the third cladding layer, wherein the second process forms a surface part of the etch-stop layer using an (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor, where 0.2xe2x89xa6x less than 0.7 and 0 less than yxe2x89xa61.
In the stated manufacturing method, the etch-stop layer has a surface part formed of an (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor (where 0.2xe2x89xa6x less than 0.7 and 0 less than yxe2x89xa61) that suffers from relatively little sublimation and is relatively unreactive with various gases. This means that the part of the etch-stop layer exposed to the high temperatures used during the formation of the third cladding layer, during the thermal cleaning, and during the formation of the current blocking layer is difficult to sublimate. This surface also has low reactivity with the gases used during these processes. As a result, an extremely flat etch stop layer can be achieved, with no metamorphosed layer being formed on its surface. This makes it possible to form the layers composing the light-confining structure on top of the etch stop layer with no decrease in the degree of crystallization, and to produce a semiconductor laser with superior laser characteristics (threshold current, slope efficiency).
The second object of the present object can also be achieved by a semiconductor laser manufacturing method, including: a first process forming,(i) a first cladding layer, (ii) an active layer on the first cladding layer, and (iii) a second cladding layer on the active layer, the second cladding layer having a different type of conductivity to the first cladding layer; a second process for forming an etch-stop layer on the second cladding layer, the etch-stop layer having a same type of conductivity as the second cladding layer; a third process for forming a current blocking layer on at least one specified region of a surface of the etch-stop layer using liquid-phase etching, the current blocking layer having a different type of conductivity to the etch-stop layer; a fourth process for performing thermal cleaning in a specified gas atmosphere after the third process has finished; a fifth process for forming a third cladding layer on regions of the surface of the etch-stop layer aside from the specified region so as to contact the current blocking layer, the third cladding layer having a different type of conductivity to the current-blocking layer, wherein the second process forms a surface part of the etch-stop layer using an (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor, where 0.2xe2x89xa6x less than 0.7 and 0 less than yxe2x89xa61.
In the stated manufacturing method, the etch-stop layer has a surface part formed of an (AlxGa1xe2x88x92x)yIn1xe2x88x92yP semiconductor (where 0.2xe2x89xa6x less than 0.7 and 0 less than yxe2x89xa61) that suffers from relatively little sublimation and is relatively unreactive with various gases. This means that the part of the etch-stop layer exposed to the high temperatures used during the formation of the current blocking layer, during the thermal cleaning, and during the formation of the third cladding layer is difficult to sublimate. This surface also has low reactivity with the gases used during these processes. As a result, an extremely flat etch stop layer can be achieved, with no metamorphosed layer being formed on its surface. This makes it possible to form the layers composing the light-confining structure on top of the etch stop layer with no decrease in the degree of crystallization, and to produce a semiconductor laser with superior laser characteristics (threshold current, slope efficiency).
Here, the third process may perform the liquid-phase etching using an etching solution including tartaric acid.
The stated manufacturing method uses an etching solution containing tartaric acid. This kind of solution has superior etching selectivity for the third cladding layer and current blocking layer. As a result, it is possible to have the etching suitably stop right at the surface of the etch-stop layer. This in turn makes it possible to form the third cladding layer and current blocking layer with the proper shape and thickness. As a result, the semiconductor laser can be made to emit light more precisely in the single lateral mode, thereby improving the laser characteristics. dr
These and other objects, advantages and features of the invention will become apparent from the following description taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. In the drawings:
FIG. 1 shows a cross-section of a semiconductor laser LS1 that is a first embodiment of the present invention;
FIGS. 2A to 2E show the processes by which the semiconductor laser LS is manufactured;
FIG. 3 is a graph showing the relationship between the composition of the etch-stop layer and the extent to which irregularities are produced in the surface of the etch-stop layer by the thermal cleaning;
FIG. 4 is a graph showing the relationship between composition of the etch-stop layer and the density (incidence) of crystal defects in the surface of the second p-type cladding layer due to the thermal cleaning;
FIG. 5 is a graph showing the relationship between the composition of the etch-stop layer and the selectivity of the etching performed on the second p-type cladding layer;
FIG. 6 shows the current-light output characteristics of semiconductor lasers; and
FIG. 7 is a cross-sectional drawing showing the construction of a conventional semiconductor laser.